The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Emissions control is one consideration in engine design and engine control. Oxides of nitrogen (NOx) are known by-products of combustion. NOx is created by nitrogen and oxygen molecules present in engine intake air disassociating in the high temperatures of combustion, with higher rates of NOx creation being associated with higher combustion temperatures and longer exposure of air molecules to the higher temperatures. NOx molecules, once created in the combustion chamber, can be converted back into nitrogen and oxygen molecules in exemplary devices known in the art within the broader category of aftertreatment devices.
Modern engine control methods utilize diverse operating strategies to optimize combustion. Some operating strategies, optimizing combustion in terms of fuel efficiency, include lean, localized, or stratified combustion within the combustion chamber in order to reduce the fuel charge necessary to achieve the work output required of the cylinder and increase engine efficiency. For example, by operating in an unthrottled condition, air intake pumping losses are reduced. While temperatures in the combustion chamber can get high enough in pockets of combustion to create significant quantities of NOx, the overall energy output of the combustion chamber, in particular, the heat energy expelled from the engine through the exhaust path can be greatly reduced from normal values. Such conditions can be challenging to exhaust aftertreatment strategies since aftertreatment devices frequently require an elevated operating temperature driven by the exhaust flow temperature to operate adequately to treat NOx emissions.
Aftertreatment devices are known to utilize chemical reactions to treat exhaust flow. One exemplary device includes a selective catalytic reduction device (SCR). Known SCR devices may utilize ammonia derived from urea injection to treat NOx. Ammonia stored on a catalyst bed within the SCR reacts with NOx, preferably in a desired proportion of NO and NO2, and produces favorable reactions to treat the NOx. One exemplary embodiment includes a preferred one to one, NO to NO2 proportion, and is known as a fast SCR reaction. Continued improvement in exhaust aftertreatment requires accurate information regarding NOx emissions in the exhaust flow in order to achieve effective NOx reduction, such as dosing proper amount of urea based on monitored NOx emissions.
Other aftertreatment devices are additionally known for treating the exhaust flow. Three way catalysts (TWC) are utilized particularly in gasoline applications. Lean NOx traps (LNT) utilize catalysts capable of storing some amount of NOx, and engine control technologies have been developed to combine these LNTs or NOx absorbers with fuel efficient engine control strategies to improve fuel efficiency and still achieve acceptable levels of NOx emissions. One exemplary strategy includes using a lean NOx trap to store NOx emissions during fuel lean operations and then purging the stored NOx during fuel rich, higher temperature engine operating conditions with three-way catalysis to nitrogen and water. Diesel particulate filters (DPF) trap soot and particulate matter in diesel applications, and the trapped material is periodically purged in high temperature regeneration events.